The present invention relates to a method of forming a seamless cylinder shell that is suitable for finishing into a seamless gas cylinder to store ultra-high purity gases at high pressure. More particularly, the present invention relates to such a method in which the cylinder shell is provided with an internal layer of nickel.
Gas cylinders are widely utilized in the art for storing gases at high pressure. Ultra-high purity gases used in the electronics industry present a particular storage problem in that corrosion product present on the inside of a gas cylinder can degrade the purity of the gas to be stored. This corrosion can be caused by the ultra-high purity gas itself if it is corrosive etching gas such as HCl.
Gas cylinders used in containing ultra-high purity gas are specially designed in order to maintain the purity of the gas by being fabricated entirely of nickel or by being formed with a layered construction having an outer layer composed of steel and an inner layer of nickel plated to the outer steel layer. As can be appreciated, gas cylinders formed solely of nickel are expensive and hence, layered construction is preferred from a cost standpoint. Additionally, pure nickel cylinders are not used where the intended service pressure exceeds 500 psig.
Nickel plated gas cylinders are constructed by cold drawing or billet piercing a steel blank to form a cylinder shell and then electroplating the inside of the cylinder shell. Thereafter, the cylinder shell is finished by spinning a cylinder head into the open end of the cylinder shell, threading the cylinder head, and heat treating the cylinder.
The drawback of nickel plated gas cylinders is that the nickel plating can contain cracks, voids and openings through which ultra-high purity gases can be contaminated or contaminants can be formed through a reaction of steel with the gas itself. In addition, the nickel plating produces a rough surface that is extremely susceptible to the retention of contaminants.
As will be discussed, the present invention solves the problems in the prior art that are attendant to the production of gas cylinders that are suitable for the storage of ultra-high purity gases at high pressure by fabricating the gas cylinder in accordance with a method of the present invention.